1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (herein after, referred to as a “TFT”) utilizing a semiconductor film (hereinafter, referred to as a “crystalline semiconductor film”) having a crystalline structure formed on a substrate and a method of manufacturing the same. In the present specification, a semiconductor device generally refers to devices that function using semiconductor characteristics. A semiconductor device manufactured according to the present invention includes a liquid crystal display device and the like having a semiconductor integrated circuit (microprocessor, signal processing circuit, high-frequency circuit, or the like) constituted by using a TFT.
2. Description of the Related Art
A liquid crystal display device having a driving circuit and a pixel portion formed on the same substrate by using a TFT is being actively manufactured. A semiconductor film is used as an active layer of a TFT, and in particular, a crystalline silicon film is used as an active layer, whereby a high field-field mobility has been realized. This technique enables a monolithic liquid crystal display device to be obtained, in which a pixel TFT constituting a pixel portion and a TFT for a driving circuit provided in the periphery of the pixel portion are formed on one glass substrate.
The electrical characteristics of a TFT depend upon the quality of a semiconductor film. In particular, a field-effect mobility depends upon the crystallinity of a semiconductor film, and directly influences response characteristics of a TFT and a display ability of a liquid crystal display device manufactured by using a TFT for a circuit.
Therefore, a method of forming a crystalline semiconductor film of good quality is being actively studied. For example, a method of forming an amorphous semiconductor film, and thereafter, crystallizing the amorphous semiconductor film by irradiation with laser light, a method of crystallizing an amorphous semiconductor film by heat treatment using an electrothermal furnace, and the like are employed. However, a semiconductor film manufactured by such a method is composed of a number of crystal grains, and its crystal orientation cannot be controlled due to its alignment in an arbitrary direction. Therefore, compared with a semiconductor of single crystal, carriers do not move smoothly, which restricts electrical characteristics of a TFT.
In contrast, Japanese Patent Application Laid-open No. Hei 7-183540 discloses a technique of crystallizing a silicon semiconductor film by adding a metal element such as nickel. Such a metal element is known to function as a catalyst to promote crystallization and lower the temperature required therefor. Such a metal element can also enhance the alignment of a crystal orientation. It is known that one kind or a plurality of kinds selected from Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au can be used as an element having a catalytic function.
However, there is a problem in that if a metal element having a catalytic function (herein, all the metal elements having a catalytic function are called a catalytic element) is added, the metal element remains in or on a semiconductor film, which varies electrical characteristics of a TFT. For example, an off current of a TFT increases to vary the electrical characteristics among respective elements. More specifically, a metal element having a catalytic function with respect to crystallization becomes unnecessary, once a crystalline semiconductor film is formed.
The inventors of the present invention disclose a method of removing a metal element added for crystallization from a particular region of a semiconductor film even at a heating temperature of about 500° C., by applying a gettering technique using phosphorus. For example, by conducting a heat treatment at 450° C. to 700° C. by adding phosphorus to a source/drain region of a TFT, a metal element added for crystallization can be easily removed from a device formation region. Japanese Patent No. 3032801 discloses an example of such a technique.
Furthermore, by using a semiconductor film of good quality having a high crystal orientation as described above, an active matrix type liquid crystal display device has been developed in which a driving circuit and a pixel portion are integrally formed on the same substrate.
A driving circuit of an active matrix type liquid crystal display device requires preventing deterioration due to a high driving ability (on-current, Ion) and hot carrier effects, whereas a pixel portion requires a low off-current (Ioff).
As a TFT structure for reducing an off current, a lightly doped drain (LDD) structure is known. In this structure, an LDD region with an impurity element added thereto in a low concentration is provided between a channel formation region and a source region or a drain region formed by adding an impurity element in a high concentration. As a structure effective for preventing deterioration of an on-current value due to hot carriers, an LDD structure in which an LDD region partially overlaps a gate electrode, i.e., a gate-drain overlapped LDD (hereinafter, referred to as a “GOLD”) structure is known.
The inventors of the present invention disclose a method of gettering a catalytic element from a semiconductor film after conducting a low-temperature crystallization process using a catalytic element as described above. For example, there are a method of forming a gettering site doped with an element (typically, phosphorus) belonging to Group 15 of the periodic table having a gettering function in a high concentration, moving a catalytic element to the gettering region by a heat treatment, and removing the gettering site, a method of gettering (moving) a catalytic element in a semiconductor layer to a source region or a drain region in the same heat treatment process as that of activation of phosphorus added to a region to be the source region or the drain region, and the like. The above-mentioned gettering enables a metal element introduced into a semiconductor film for crystallization to be removed by conducting a heat treatment at 550° C. for about 4 hours.
However, the concentration of phosphorus added to a semiconductor film for obtaining a gettering function is 1×1020/cm3 or more, preferably 1×1021/cm3. Thus, it takes a long time for doping the semiconductor film with phosphorus.
Furthermore, addition of phosphorus in a high concentration by ion implantation or ion doping (in the present specification, which refers to a method in which mass separation of ions to be implanted is not conducted) makes it difficult for a semiconductor film to be recrystallized.
Furthermore, in an active matrix type liquid crystal display device in which a driving circuit is integrally formed, performance required for a driving circuit is different from that required for a pixel portion. Therefore, if it is attempted to optimize the structure of a TFT in accordance with the respective requirements, production processes become complicated, which necessarily increases the number of required photomasks. On the other hand, according to a procedure of forming a region containing an impurity element, such as an LDD region in a self-alignment manner by using a gate electrode, a processing precision is inescapably worsened along the enlargement in a substrate size.